In various industries or arts, it is necessary, as part of the overall process, to dry a continuous sheet or strip of material. For example, in the art of paper-making, the relatively wet or damp strip or web of paper fibers is often passed over and mostly about a rotatable drum or cylinder assembly which is heated in order to thereby evaporate the moisture of the wet or damp paper sheet in contact with and passing about the outer cylindrical surface of such rotatable drum or cylinder assembly.
The prior art has employed steam for the heating of such rotatable drying drums or cylinder assemblies. More specifically, heretofore, the prior art has employed a drying cylinder or drum assembly comprised of a cylindrical shell having axially closed end-walls defining an inner chamber into which, as through the axis of rotation in an end wall, steam is supplied in order to sufficiently heat the cylindrical shell, and the outer cylindrical surface thereof, and thereby achieve the desired degree of drying of the paper sheet or material passing in contact with the said outer cylindrical surface.
The steam thusly admitted into the inner chamber of the drying cylinder or drum assembly, upon giving-up some of its heat, forms a condensate within said inner chamber and, obviously, such condensate must be removed in order to maintain a continuous drying process.
Heretofore, the prior art as disclosed, for example, by Austria Letters Patent No. 244,318, has proposed the use of a suction type conduit means for the removal of condensate from the interior of such a steam heated rotatable drying cylinder assembly. In such a proposed prior art apparatus, the pressure generally downsteam of the condensate-removing conduit means is maintained at a magnitude less than the pressure within the interior of the drying cylinder assembly and the steam, to the interior of the cylinder assembly, is supplied in such a volume and under such a pressure as to result in a portion of such steam flowing as a high speed stream through a gap or clearance formed between the surface of the film of condensate and the juxtaposed rim of an inlet leading to and cooperating with the suction conduit means. The purpose of creating such a high speed stream was to cause the stream to entrain therein part of the liquid forming the film of condensate. Such a prior art proposed condensate removing system was based on the assumption that the condensate would, during comparatively high peripheral velocities of the drying cylinder assembly, form a continuous inner cylindrical ring or film of constant thickness on the inner surface of the cylindrical shell. Such a prior art proposed condensate-removing system was found to be generally acceptable even where the peripheral velocity of the drying cylinder assembly was so low as to result in a sump, puddle or pool of condensate in the lower portion of the interior of the drying cylinder assembly.
However, especially in the paper making art, the working speeds of such drying cylinder assemblies have undergone, in the recent past, steady increases as to thereby obtain greater rates of paper production. It has been found that in such instances, where significantly higher rotational velocities are experienced by the drying cylinder assemblies, a recognizable continuous strip-like pattern is formed in the paper sheet. Such strip pattern has been found to be, generally, over-dried as compared to the remainder of the stock forming the paper sheet. Further, it has been found that such strip pattern is produced in the paper sheet at a location which corresponds to the axial position, along the axis of the drying cylinder assembly, at which the inlet end or intake head of the condensate-removing conduit means is situated.
It has been found that at such relatively high rotational speeds, the remaining condensate (not removed by the intake head or inlet structure) forms a spurting and turbulent zone behind the intake head or inlet structure. As a consequence of such turbulent zone, in the condensate layer remaining behind the inlet structure, the rate of heat transfer, in that zone, from the interior to the exterior cylindrical surface of the drying cylinder assembly is significantly increased causing the outer cylindrical surface to have a ring-like annular area of a temperature which is too hot and such annular area then has the effect of comparatively over-drying the portion of the paper sheet coming in contact therewith.
Heretofore, it has been proposed by the prior art that in order to overcome such a problem of an over-heated zone on the drying cylinder one should employ an intake head or inlet structure sometimes referred to as a "peel-syphon" type and disclosed, for example, in Federal Republic of Germany Patent Office Publication No. AS-29-03-170. In such a prior art inlet structure the inlet opening thereof has a somewhat peeling action on the film or layer of condensate. The kinetic energy of such peeled portion of the condensate partly assists the flow of such peeled condensate into and through the associated condensate-removing conduit means. However, the major motivating and removing force of such peeled condensate is the transporting and entraining effect of a high speed stream of steam flowing through a gap or clearance formed between the surface of the film of condensate and the juxtaposed edge or rim of the inlet opening of the inlet structure (much as disclosed by said Austria Letters Patent No. 244,318). One of the serious disadvantages of such a prior art inlet structure is that there is a comparatively high usage of steam; further, the thickness of the remaining condensate film or layer is comparatively large resulting in the rate of heat transfer to the outer cylindrical surface of the dryer cylinder assembly being reduced from the desired rate. Still further, such a "peel-syphon" type of structure proposed by the prior art is capable of removing at most only a small portion of the condensate from the drying cylinder when such drying cylinder is used in a drying process employing slow working speeds resulting in the formation of a condensate sump or pool in the lower part of the interior of the drying cylinder.
Other prior art attempts at eliminating the creation of such an overly-heated zone on the drying cylinder have not been found to be acceptable. One suggestion was to form an inner circumferential groove within the inner surface of the drying cylinder with such groove being positioned generally in a plane passing normal to the axis of rotation of the drying cylinder and passing through the medial portion of the intake head or inlet structure. Such was found not to correct the problem of creating an over-heated zone.
The prior art also suggested making the drying cylinder axially overly-long as to be significantly axially longer than the width of the paper sheet to be passed thereagainst and then placing the condensate intake head or inlet structure at a position as to be situated axially beyond the edge of the paper sheet. This has not been found acceptable.
The prior art has also suggested that a plurality of drying cylinder assemblies, arranged in series, be employed for collectively drying the paper sheet sequentially engaging such drying cylinders. More specifically, the prior art contemplated positioning the respective condensate intake heads or inlet structures at differing axial locations as to, in effect, created respective overly-heated zones which would not be aligned with each other and then partially but unevenly drying the paper sheet as it passes against each drying cylinder with the hope that thereby, through such overlapping cumulative drying the finally dried paper sheet would not exhibit the undesired overly-dried strip therein.
The prior art has also proposed preventing the creation of an overly-heated zone, on the dryer cylinder, by employing a heat insulating ring on the inner cylindrical surface of the drying cylinder as to be passing (during rotation of the dryer cylinder) in juxtaposition to the condensate intake head or inlet structure. Such a prior art structure, as disclosed generally by Federal Republic of Germany Patent Office Publication No. OS-29-30-985, has not been found acceptable.
Further, the prior art has proposed the use of rotating, instead of stationary, condensate intake heads or inlet structures in order to prevent the creation of an overly heated zone on the drying cylinder. However, this has not been found acceptable especially in view of the dramatically increased pressure differential and steam consumption necessary to overcome the centrifugal force tending to prevent the condensate from flowing through the associated conduit means toward the axis of rotation.
The invention as herein disclosed and described is primarily directed to the solution of the aforestated as well as other related and attendant problems of the prior art.